Modular building structure elements of slabs with central support posts

ABSTRACT

A prefabricated element for modular building structures which comprises a column carrying at its top end a slab composed of a ceiling plate and a grid of joists integral with the ceiling plate and extending upwardly therefrom. The slab resembles an upside-down waffle type floor. The column is connected to a central portion of the slab such that the ceiling plate, which is at the bottom of the slab, takes compressive stresses and the joists, which are above the ceiling plate, take tensile stresses. The bottom end of the column is provided with a downwardly extending pin, and the slab is provided at its center with an opening adapted to receive the pin to allow for convenient and accurate vertical stacking of pluralities of structural elements. The slab may be square or rectangular, and may be provided at its lateral sides with connecting elements for attaching together adjacent slabs of different structural elements to form a load bearing building structure. A structural element comprising a column and a slab integral therewith may be prefabricated and transported to the construction site in one piece. Alternately, a modified column having a mushroom-shaped cap, and a modified slab having a conforming opening adapted to receive the mushroom cap, can be preformed separately, and may be transported to the construction site separately and assembled there.

United StatesPatent 1 Arnold 11 3,788,012 Jan. 29, 1974 11/1963 Switzerland 52/587 Primary Examiner-Frank L. Abbott Assistant Examiner-Les1ie A. Braun [5 7] ABSTRACT A prefabricated element for modular building structures which comprises a column carrying at its top end a slab composed of a ceiling plate and a grid of joists integral with'the ceiling plate and extending upwardly therefrom. The slab resembles an upside-down waffle type floor. The column is connected to a central portion of the slab such that the ceiling plate, which is at the bottom of the slab, takes compressive stresses and the joists, which are above the ceiling plate, take tensile stresses. The bottom end of the column is provided with a downwardly extending pin, and the slab is provided at its center with an opening adapted to receive the pin to allow for convenient and accurate vertical stacking of pluralities of structural elements. The slab may be square or rectangular, and may be provided at its lateral sides with connecting elements for attaching together adjacent slabs of different structural elements to form a load bearing building structure. A structural element comprising a column and a slab integral therewith may be prefabricated and transported to the construction site in one piece. Altemately, a modified column having a mushroomshaped cap, and a modified slab having a conforming opening adapted to receive the mushroom cap, can be preformed separately, and may be transported to the construction site separately and assembled there.

16 Claims, 8 Drawing Figures MODULAR BUILDING STRUCTURE ELEMENTS OF SLABS WITH CENTRAL SUPPORT POSTS [75] Inventor: Steven Arnold, Westfield, NJ.

[73] Assignee: Arnold Associates, Inc., Westfield,

[22] Filed: Feb. 22, 1972 [21] Appl. No.: 227,855

Related US. Application Data [63] Continuation-impart of Ser. No. 71,636, Sept. 14,

1970, abandoned.

[52] U.S. Cl 52/73, 52/251, 52/263 [51] Int. Cl E04b 5/43 [58] Field of Search 52/263, 300, 301, 73, 251,

[56] References Cited UNlTED STATES PATENTS 3,152,421 10/1964 Middendorf 52/126 2,171,338 8/1939 Henderson 52/220 2,783,638 3/1957 Henderson 52/723 1,031,044 7/1912 Conzelman 52/723 1,970,457 8/1934 Jenkinson....

3,495,367 2/1970 Kobayashi....

371,845 10/1887 Jackson 3,543,458 12/1970 Guritz 2,215,773 9/1940 Workman....

3,354,593 11/1967 Zukas 3,383,816 5/1968 Hodson 52/263 FOREIGN PATENTS OR APPLICATlONS 1,117,843 11/1961 Germany 52/73 1,061,783 3/1967 Great Britain... 52/263 1,012,278 12/1965 Great Britain 52/263 1 10,920 5/1964 Czechoslovakia 52/263 MODULAR BUILDING STRUCTURE ELEMENTS OF SLABS WITH CENTRAL SUPPORT POSTS CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of a copending application of the same inventor entitled Modular Building System, Ser. No. 71,636, filed on Sept. 14, 1970, now abandoned.

BACKGROUND OF THE INVENTION The invention is in the field of standardized structural elements which are cast prior to construction and can be used to construct a variety of buildings at short construction time and at reasonable expense. There are prior art systems providing precast standardized structural elements of this general type. One example is described in Engineering News-Record, pages 20, 21, June 25, 1970, and consists of five basic precast reinforced concrete components: columns, capital slabs, floor slab, cornices and girder-slabs. The components are dimensioned in multiples of a basic module and a contractor can fit and tie the individual elements of the system together to form the building frame. Other examples of structural elements for similar systems are Middendorf US. Pat. No. 3,l52,42l and Zukas US. Pat. No. 3,354,593.

To minimize costs of construction and to minimize construction time, it is desirable that: the structural elements for such structures be easy to manufacture, have minimum weight for given load-carrying capability, provide for easy assembly into a building structure, and require minimum of finishing work to complete a building constructed therefrom. Some of these design goals are conflicting and compromises must be made. Thus, for example, the Engineering News Record System, supra, describes a system where the components are easy to manufacture, but the floors in a building constructed therefrom are not even because the girder slabs, which are some 6 inches thick, overlap only the margins of the capital slabs, and there is a 6-inch drop between girder and capital slab surfaces. In the case of the Middendorf structural element, which comprises a floor or roof slab and a column supporting the slab at the center, the slab is an open grid which does not provide for optimum distribution of tensile and compressive stresses, and which requires the addition of both ceiling and floor surfaces. Similarly, the Zukas patent shows column-slab structural elements similar to the ones shown in Middendorf, but comprising a solid concrete slab which again does not provide for optimum distribution of tensile and compressive stresses to minimize slab weight for a given load capacity. Additionally, the systems disclosed in the references discussed above make no convenient provisions for accommodating into the horizontal slab various horizontally running conduits and wiring.

SUMMARY OF THE INVENTION In one form, the invented structural element is integrally formed of reinforced concrete and comprises a column integral with a rectangular or square horizontal slab. The slab resembles an upside-down waffle type floor, with the continuous ceiling plate at the bottom of the horizontal slab and with the joists extending up- 5 wardly therefrom and forming a rectangular or square grid. The top end of the column is integral with the central portion of the slab. The free portions of the joists, i.e., the portions between the nodes where joists intersect, are provided with openings to allow for passing various conduits and wiring. A downwardly extending pin is provided at the bottom of each column, and a matching upwardly facing opening is provided at the center of each slab to allow for convenient and accurate vertical stacking of structural elements and for resisting forces normal to the column length. The lateral sides of the slabs are provided with connecting elements to allow for tieing together separate slabs arranged side-by-side in a finished structure.

In another form, the invented structural element is preformed in two separate pieces, namely, a column having a laterally extending cap and a slab provided with a downwardly facing opening adapted to receive the column cap, and the two pieces are later joined to form the complete structural element.

In still another form, the joists are not of reinforced concrete, but are steel I-beams whose lower flanged portions are embedded in the ceiling plate.

One object of the invention is to provide a structural element for modular building structures which can be quickly and conveniently manufactured, and which is susceptible to manufacturing by mass production methods using high degree of mechanization and automation to achieve low cost year round production. The structural element can include either reinforced concrete or steel, depending on considerations of costs, desired weight-load ratios and the like.

Another object of the invention is to provide structural elements which can be quickly and conveniently assembled into a finished structure, with commonly used and commonly available construction equipment, providing high degree of design freedom, and providing interior space which is substantially unobstructed and thus allows freedom of interior design and interior change.

Another object of the invention is to provide structural elements which can be assembled into a structure that can be later conveniently disassembled, moved to another location and again assembled, either into the same structure or into a different structure.

Another object of the invention is to provide structural elements which allow for convenient, fast and inexpensive installation and alteration of wiring and conduits, and which provide superior fire and sound insulation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of the invented structural element.

FIG. 2 is a sectional view taken along line 22 of FIG. 1 and showing a slab and a column which are integrally formed.

FIG. 3 is a partial sectional view taken along line 3-3 of FIG. 1 and showing one possible embodiment of a connecting element for connecting slabs in side-toside relationship.

FIG. 4 is a partial sectional view taken along line 44 of FIG. 1 and showing one possible embodiment of a connecting element matching the connecting element shown. in FIG. 3.

FIG. 5 is a partial sectional view similar to the views of FIGS. 3 and 4 and showing the connecting elements of FIGS. 3 and 4 tieing together two adjacent slabs in a side-to-side relationship.

FIG. 6 is a sectional view similar to the view of FIG. 2 but showing a slab having steel I-beams as joists.

FIG. 7 is a partial plan view of the central portion of a modified slab similar to that shown in FIG. 1 but adapted to be preformed separately from the column and to be connected thereto subsequently.

FIG. 8 is a sectional view taken along line 8 -8 of FIG. 7 and showing the modified slab of FIG. 7 and a modified column.

Detailed Description Referring to FIGS. 1 and 2, the invented structural element comprises a slab l0 and a column 12 which are formed integrally of reinforced concrete. The slab comprises a substantially continuous ceiling plate 14, which is at the bottom of the slab l0, and a grid of joists 16 formed integrally with the ceiling plate 14 and extending upwardly therefrom. It is noted that this is the reverse of the waffle type slab floor where the joists extend downwardly from the floor, and that the slab l0 resembles an upside-down waffle type slab floor. The ceiling plate 14 may be reinforced with a welded steel mesh 20, or otherwise, and each of the joists 16 may have a reinforcing rod 22, which is preferably in the upper portion of the joists l6, and possibly another reinforcing rod 24 in the lower portion of the joists 16. The reinforcing rods 22 and 24 may be prestressed. The column 12 also includes a con-ventional reinforcing structure 26.

The slab 10 may be square, as shown in FIG. I, or it may have another convenient shape, such as a rectangle, a hexagon, etc. Similarly, the grid of joists 16 may be a square grid of joists l6 intersecting at right angles, as shown in FIG. 1, or it may have a different configuration, i.e., it may be a rectangular grid, or a grid of joists intersecting at other than right angles. What is important for the subject invention is that the slab 10 have at its bottom a substantially continuous ceiling plate 14 which is designed to be the primary bearer of compressive stresses, and that the joists 16 extend upwardly from the ceiling plate 14 to be the primary bearer of tensile stresses. As noted earlier, this is the reverse of the conventional waffle type floor arrangement.

The ceiling plate 14 is called substantially continuous to mean that it should be continuous, except possibly for various openings for conduits, wires, fixtures and the like. Since the purpose of the ceiling plate 14 is to be the primary bearer of compressive stresses, discontinuities in it are detrimental to its strength in resisting compressive stresses.

For the purposes of providing for stacking structural elements on top of each other and for resisting forces normal to the column length, each column 12 is provided with a pin 28 partly embedded in the bottom portion of the column 12 and extending downwardly therefrom, while each slab 10 is provided at its center with a matching upwardly facing opening 30 defined by a hollow sleeve 32 embedded in the slab 10. The central square of the grid ofjoists is shown in FIGS. 1 and 2 as solid reinforced concrete, but it should be noted that this is not essential. That is, the central square may be like the other squares and the hollow sleeve 32 may be embedded in the column 12. The hollow sleeve 32 is merely for convenience in defining an opening 30 of a proper size and shape; the opening 30 may be otherwise defined.

It should be noted here that directional terms such as horizontal, vertical, lateral and the like are used merely for convenience to denote the orientation of various surfaces as they would appear in a structural element when it forms a part of a building.

Generally, a plurality of structural elements are joined in a side-by-side relationship to form the floor of a building. Means are provided therefore for tieing together adjacent slabs l0 conveniently and quickly. To this end, the ceiling plate 14 extends at two lateral sides of the slab l0 outwardly of the outermost joists 16 to form a lip 38, and the ceiling plate 14 is recessed at the two other lateral sides of the slab 10 by a distance equal to the distance the lip 38 extends outwardly. At the slab sides which have the ceiling plate lip 38, the outermost joists 16 are provided with vertical grooves 34 located opposite the places where the outermost joists 16 are joined by other joists 16. A male connecting element 42 provided with an upwardly extending bolt shank 44 is embedded in the lower portion of the outermost joists 16 at each of the vertical grooves 34.

Similarly, the two lateral sides of the slab 10 where the ceiling plate 14 is recessed are provided with vertical grooves 36 opposite the places where the outermost joists 16 are joined by other joists, and a female connecting element 40 is embedded in the lower portion of the outermost joists 16 at each vertical groove 36'. The female connecting member 40 is provided with an aperture 40a adapted to receive the shank of the bolt 44 in the male connecting member 42. The tieing means described above are one example of means for joining together adjacent structural elements; other means are possible for carrying out the same function.

When adjacent slabs 10 are to be joined together, that lateral side of one slab 10 which has the ceiling plate lip 38 is brought in engagement with that lateral side of another slab 10 at which the ceiling plate 14 is recessed, as shown in FIG. 5. In the position shown in FIG. 5, the aperture 40a in the female connecting element 40 is aligned with and receives the shank in the bolt 44, and a nut 44a is used to afi'ix the male connecting element 42 to the female connecting element 40. Then, grout may be poured into the opening formed by each pair of facing vertical grooves 34 and 36, and a reinforcing rod 46, which may be suitably tensioned, may be placed between the facing outermost joists 16 of the two adjacent slabs 10.

It is noted that there may be alternative arrangements for tieing together adjacent slabs 10. For example, there may be slabs 10 of one type which have ceiling plates 14 with ceiling plate lips 38 on all four lateral sides of the slab l0, and there may be slabs 10 of another type, namely, having ceiling plates 14 which are correspondingly recessed on all four lateral sides of the slab.

The slabs 10 shown in FIGS. 1 through 5 have joists 16 which are integrally formed of reinforced concrete. FIG. 6 shows joists of a different form, namely, joists formed of steel I-beams whose lower flanged portion is embedded in the ceiling plate 14. In particular, FIG. 6 shows a structural element composed of a column 12 which is identical with the column of the structural element shown in FIGS. 1 through 5, but which has steel l-beams 48 serving as joists. The lower flanged portion 48a of each steel beam 48 is embedded in the ceiling plate 14. The ceiling plate 14 is identical to that shown in FIGS. 1 through 5. The central square of the slab shown in FIG. 6 may be identical to the central square of the slab 10 shown in FIGS. 1 through 5.

For the purpose of facilitating the passing of various conduits and wiring, and thereby reducing costs and construction time, each of the joists 16 may be provided with openings 50 in the portions which are between the nodes where intersecting joists join. All piping, plumbing, conduits and ducts can run horizontally through the openings 50 in the joists 16 to be thus between the floor and the ceiling of a finished floor, and can come up through the floor or down through the ceiling at any desired point, independently from the floors above or the ceiling below. Due to the openings 50 in the joists 16, there should be no additional work in breaking through barriers such as walls and joists in installing conduits and wiring, and time and costs can be saved.

When it is inconvenient or undesirable to manufacture or to transport structural elements in which the slabs l0 and the columns 12 are integral with each other, similar advantages may be obtained from slabs 52 and columns 54 constructed as shown in FIGS. 7 and 8. The column 54 is similar to the column 12 shown in FIGS. 1 through 6, but has at its upper end a mushroom shaped cap 56 which is square in horizontal cross-section and which diminishes in cross-sectional area in the upward direction. The cap 56 may be of different suitable shapes, e.g., it may be of circular, rectangular, or other suitable cross-section. The slab 52 is similar to the slab 10 shown in FIGS. 1 through 6, except that its central grid square is provided with an opening 58 conforming to the shape of the column cap 56 and adapted to receive it. The slabs 52 and columns 54 may be manufactured separately, and can be later joined into an integral structural element by placing the slab 52 on top of the column cap 56 as shown in FIG. 8, and pouring grout into the space'between'the column cap 56 and the opening 58 in the slab 52. The column 54 is provided at its top end with an upwardly facing opening 30 defined by the sleeve 32 embedded in the column 54. The opening 30 receives a pin 28 for vertical stacking of structural elements, and for resisting forces normal to the columns length. Reinforcing rods 60 may be embedded in the walls 62 defining the central opening in the slab 10 which receives the column cap 56.

A possible variation of the structural element shown in FIGS. 1 through 6 is to arrange the column 12 and the slab 10 such that the slab 10 is intermediate the longitudinal ends of the column 12. For example, onethird of the column 12 may extend above the slab 10. Similarly, the structural element shown in FIGS. 6 and 7 may include a column 54 which has its cap portion 56 intermediate its longitudinal ends.

I claim:

1. A modular building structure including a structural element comprising an integral horizontal slab having a substantially continuous, compressive stress bearing, horizontal, relatively thin, reinforced ceiling plate and a tensile stress bearing grid of intersecting, relatively thin and spaced apart joists integral with the ceiling plate and extending upwardly therefrom, each of at least a plurality of said joists formed with a plurality of openings intermediate the locations where joists intersect to allow for the passing of conduits through said openings and including stress reinforcing means extending along the upper marginal portion of the joist above said openings therein, with a central grid portion of the slab being at least partly filled in, and a vertical load-bearing column connected to said central filled in portion of the slab for supporting the slab.

2. A structure as in claim 1 wherein the slab and the column are in a rigid connection.

3. A structure as in claim 1 wherein the slab and the column are formed integrally of reinforced concrete.

4. A structure as in claim 1 wherein the joists are formed of metal I-beams which extend longitudinally across the slab and whose lower flanged portions are embedded in the ceiling plate. 1

5. A structure as in claim 1 wherein the openings in each joist form a major portion of the joist area in a longitudinal section thereof.

6. A structure as in claim 1 wherein the column includes a pin extending downwardly from its bottom end, and the slab is formed with an upwardly facing opening at its central portion adapted to receive said pin for vertical stacking of a plurality of structural elements.

7. A structure as in claim 1 wherein the column includes an integrally formed cap portion extending laterially of the column and diminishing incrosssection in the upward direction, and the slab is formed with a central opening conforming to the column cap shape and adapted to receive the cap.

8. A structure as in claim 7 wherein the joists grid is a square grid and the slab opening adapted to receive the column cap is within the central square of the grid.

9. A modular building structure comprising at least a first and a second structural element each comprising an integral horizontal slab formed of a compressive load bearing, relatively thin, reinforced ceiling plate and a tensile load-bearing grid of intersecting, relatively thin and spaced apart joists integral with the ceiling plate and extending upwardly from the ceiling plate, each of at least a plurality of said joists formed with a plurality of openings intermediate the locations where joists intersect to allow for the passing of conduits through said openings and including stress reinforcing means extending along the upper marginal portion of the joist above said openings therein, a first load-bearing vertical column structurally connected with a central grid portion of the first slab for supporting the first slab and a second load-bearing vertical column structurally connected with a central grid portion of the second slab for supporting the second slab, and means for structurally connecting the first and second structural elements with each other to form said building structure.

10. A structure as in claim 9 wherein each of the slabs includes tieing means located along at least one lateral marginal portion thereof, wherein the first and second slabs are disposed laterally adjacent each other, with the ceiling plates at the same horizontal plane, and wherein the connecting means includes means for rigidly connecting said tieing means of the first and second slabs.

11. A structure as in claim 9 wherein the bottom end of the first vertical column and the top end of the second vertical column are each provided with matching tieing means and the first column is stacked on top of the second column with said tieing means engaging each other, and wherein the connecting means include means for connecting the tieing means of said first and second columns.

12. A structure as in claim 11 wherein the tieing means of the first column comprises a downwardly extending pin and the tieing means of the second column includes a recess within the top end of said second column for receiving said downwardly extending pin.

13. A structural element for a modular building structure comprising an integral horizontal slab having a substantially continuous, compressive stress bearing, relatively thin and reinforced horizontal ceiling plate and a tensile stress bearing grid of intersecting, rela tively thin and spaced apart joists integral with the ceiling plate and extending upwardly therefrom, with each joist extending horizontally between two opposite lateral ends of the ceiling plate and each of at least a plurality of said joists formed with a conduit receiving opening in each of at least a plurality of the joist portions which are between adjacent intersecting joists and including stress reinforcing means extending along the upper marginal portion of the joist above said openings therein, and a vertical load-bearing column structurally connected with a central grid portion of the slab for supporting the slab with the ceiling plate disposed horizontally and below the joists.

14. A structural element as in claim 13 wherein the joists are formed of metal l-beams which extend longitudinally across the slab and whose lower flanged portions are embedded in the ceiling plate.

15. A structural element as in claim 13 wherein each of said joists is formed with an oblong horizontal opening in each portion which is between two adjacent intersecting joists, said openings forming a major portion of the joist area in a vertical plane.

16. A structural element as in claim 13 wherein the grid of intersecting joists is a rectangular grid and each of the joist portions forming each of the four sides of a grid rectangle is formed with an opening therein for passing conduits therethrough.

* a t: a: a: 

1. A modular building structure including a structural element comprising an integral horizontal slab having a substantially continuous, compressive stress bearing, horizontal, relatively thin, reinforced ceiling plate and a tensile stress bearing grid of intersecting, relatively thin and spaced apart joists integral with the ceiling plate and extending upwardly therefrom, each of at least a plurality of said joists formed with a plurality of openings intermediate the locations where joists intersect to allow for the passing of conduits through said openings and including stress reinforcing means extending along the upper marginal portion of the joist above said openings therein, with a central grid portion of the slab being at least partly filled in, and a vertical load-bearing column connected to said central filled in portion of the slab for supporting the slab.
 2. A structure as in claim 1 wherein the slab and the column are in a rigid coNnection.
 3. A structure as in claim 1 wherein the slab and the column are formed integrally of reinforced concrete.
 4. A structure as in claim 1 wherein the joists are formed of metal I-beams which extend longitudinally across the slab and whose lower flanged portions are embedded in the ceiling plate.
 5. A structure as in claim 1 wherein the openings in each joist form a major portion of the joist area in a longitudinal section thereof.
 6. A structure as in claim 1 wherein the column includes a pin extending downwardly from its bottom end, and the slab is formed with an upwardly facing opening at its central portion adapted to receive said pin for vertical stacking of a plurality of structural elements.
 7. A structure as in claim 1 wherein the column includes an integrally formed cap portion extending laterially of the column and diminishing in cross-section in the upward direction, and the slab is formed with a central opening conforming to the column cap shape and adapted to receive the cap.
 8. A structure as in claim 7 wherein the joists grid is a square grid and the slab opening adapted to receive the column cap is within the central square of the grid.
 9. A modular building structure comprising at least a first and a second structural element each comprising an integral horizontal slab formed of a compressive load bearing, relatively thin, reinforced ceiling plate and a tensile load-bearing grid of intersecting, relatively thin and spaced apart joists integral with the ceiling plate and extending upwardly from the ceiling plate, each of at least a plurality of said joists formed with a plurality of openings intermediate the locations where joists intersect to allow for the passing of conduits through said openings and including stress reinforcing means extending along the upper marginal portion of the joist above said openings therein, a first load-bearing vertical column structurally connected with a central grid portion of the first slab for supporting the first slab and a second load-bearing vertical column structurally connected with a central grid portion of the second slab for supporting the second slab, and means for structurally connecting the first and second structural elements with each other to form said building structure.
 10. A structure as in claim 9 wherein each of the slabs includes tieing means located along at least one lateral marginal portion thereof, wherein the first and second slabs are disposed laterally adjacent each other, with the ceiling plates at the same horizontal plane, and wherein the connecting means includes means for rigidly connecting said tieing means of the first and second slabs.
 11. A structure as in claim 9 wherein the bottom end of the first vertical column and the top end of the second vertical column are each provided with matching tieing means and the first column is stacked on top of the second column with said tieing means engaging each other, and wherein the connecting means include means for connecting the tieing means of said first and second columns.
 12. A structure as in claim 11 wherein the tieing means of the first column comprises a downwardly extending pin and the tieing means of the second column includes a recess within the top end of said second column for receiving said downwardly extending pin.
 13. A structural element for a modular building structure comprising an integral horizontal slab having a substantially continuous, compressive stress bearing, relatively thin and reinforced horizontal ceiling plate and a tensile stress bearing grid of intersecting, relatively thin and spaced apart joists integral with the ceiling plate and extending upwardly therefrom, with each joist extending horizontally between two opposite lateral ends of the ceiling plate and each of at least a plurality of said joists formed with a conduit receiving opening in each of at least a plurality of the joist portions which are between adjacent intersecting joists and including stress reinforcing means extending along the upper marginal portion of the joist above said openings therein, and a vertical load-bearing column structurally connected with a central grid portion of the slab for supporting the slab with the ceiling plate disposed horizontally and below the joists.
 14. A structural element as in claim 13 wherein the joists are formed of metal I-beams which extend longitudinally across the slab and whose lower flanged portions are embedded in the ceiling plate.
 15. A structural element as in claim 13 wherein each of said joists is formed with an oblong horizontal opening in each portion which is between two adjacent intersecting joists, said openings forming a major portion of the joist area in a vertical plane.
 16. A structural element as in claim 13 wherein the grid of intersecting joists is a rectangular grid and each of the joist portions forming each of the four sides of a grid rectangle is formed with an opening therein for passing conduits therethrough. 